[virtio-comment] Re: [PATCH V2 1/6] virtio: introduce virtqueue state

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 03, 2023 at 06:34:32PM +0800, Zhu Lingshan wrote:
> This patch adds new virtqueue facility to save and restore virtqueue
> state. The virtqueue state is split into two parts:
> 
> - The available state: The state that is used for read the next
>   available buffer.
> - The used state: The state that is used for make buffer used.
> 
> This will simply the transport specific method implementation. E.g two
> le16 could be used instead of a single le32). For split virtqueue, we
> only need the available state since the used state is implemented in
> the virtqueue itself (the used index). For packed virtqueue, we need
> both the available state and the used state.
> 
> The typical use cases are live migration and debugging.
> 
> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  content.tex     |  7 ++++--
>  packed-ring.tex | 58 +++++++++++++++++++++++++++++++++++++++++++++++++
>  split-ring.tex  | 39 +++++++++++++++++++++++++++++++++
>  3 files changed, 102 insertions(+), 2 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index 0a62dce..76813b5 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -99,10 +99,10 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B
>  \begin{description}
>  \item[0 to 23, and 50 to 127] Feature bits for the specific device type
>  
> -\item[24 to 41] Feature bits reserved for extensions to the queue and
> +\item[24 to 42] Feature bits reserved for extensions to the queue and
>    feature negotiation mechanisms
>  
> -\item[42 to 49, and 128 and above] Feature bits reserved for future extensions.
> +\item[43 to 49, and 128 and above] Feature bits reserved for future extensions.
>  \end{description}
>  
>  \begin{note}
> @@ -872,6 +872,9 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>  	\ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for
>  	handling features reserved for future use.
>  
> +  \item[VIRTIO_F_QUEUE_STATE(42)] This feature indicates that the device allows the driver
> +  to access its internal virtqueue state.
> +
>  \end{description}
>  
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> diff --git a/packed-ring.tex b/packed-ring.tex
> index 9eeb382..ad6aba0 100644
> --- a/packed-ring.tex
> +++ b/packed-ring.tex
> @@ -729,3 +729,61 @@ \subsection{Receiving Used Buffers From The Device}\label{sec:Basic Facilities o
>          process_buffer(d);
>  }
>  \end{lstlisting}
> +
> +\subsection{Virtqueue State}\label{sec:Basic Facilities of a Virtio Device / Packed Virtqueues / Virtqueue State}
> +
> +When VIRTIO_F_QUEUE_STATE has been negotiated, the driver can set and
> +get the device internal virtqueue state through the following
> +fields. The implementation of the interfaces is transport specific.
> +
> +\subsubsection{\field{Available State} Field}
> +
> +The available state field is two bytes of virtqueue state that is used by
> +the device to read the next available buffer. It is presented in the followwing format:
> +
> +\begin{lstlisting}
> +le16 {
> +  last_avail_idx : 15;
> +  last_avail_wrap_counter : 1;
> +};
> +\end{lstlisting}
> +
> +The \field{last_avail_idx} field is the free-running location
> +where the device read the next descriptor from the virtqueue descriptor ring.
> +
> +The \field{last_avail_wrap_counter} field is the last driver ring wrap
> +counter that was observed by the device.
> +
> +See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}.
> +
> +\subsubsection{\field{Used State} Field}
> +
> +The used state field is two bytes of virtqueue state that is used by
> +the device when marking a buffer used. It is presented in the followwing format:
> +
> +\begin{lstlisting}
> +le16 {
> +  used_idx : 15;
> +  used_wrap_counter : 1;
> +};
> +\end{lstlisting}
> +
> +The \field{used_idx} field is the free-running location where the device write the next
> +used descriptor to the descriptor ring.
> +
> +The \field{used_wrap_counter} field is the wrap counter that is used
> +by the device.
> +
> +See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}.
> +
> +\devicenormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Packed Virtqueues/ Virtqueue State}
> +
> +The device SHOULD only accept setting Virtqueue State of any packed virtqueues when DRIVER_OK is not set in \field{device status}, or SUSPEND is set in \field{device status}.

Except SUSPEND is undefined and is explained in patch 2 :(
Please do not split patches like this, you are just
splitting them at random boundaries.


> +Otherwise the device MUST ignore any writes to Virtqueue State of any packed virtqueues.
> +
> +When SUSPEND is set, the device MUST record the Virtqueue State of every enabled packed virtqueue
> +in \field{Available State} field and \field{Used State} field respectively,
> +and correspondingly restore the Virtqueue State of every enabled packed virtqueue
> +from \field{Available State} field and \field{Used State} field when DRIVER_OK is set.
> +
> +The device SHOULD reset \field{Available State} field and \field{Used State} field upon a device reset.
> diff --git a/split-ring.tex b/split-ring.tex
> index de94038..a78b44d 100644
> --- a/split-ring.tex
> +++ b/split-ring.tex
> @@ -734,3 +734,42 @@ \subsection{Receiving Used Buffers From The Device}\label{sec:Basic Facilities o
>  }
>  \end{lstlisting}
>  \end{note}
> +
> +\subsection{Virtqueue State}\label{sec:Basic Facilities of a Virtio Device / Splited Virtqueues / Virtqueue State}
> +
> +When VIRTIO_F_QUEUE_STATE has been negotiated, the driver can set and
> +get the device internal virtqueue state through the following
> +fields. The implementation of the interfaces is transport specific.
> +
> +\subsubsection{\field{Available State} Field}
> +
> +The available state field is two bytes of virtqueue state that is used by
> +the device to read the next available buffer. It is presented in the followwing format:
> +
> +\begin{lstlisting}
> +le16 last_avail_idx;
> +\end{lstlisting}
> +
> +The \field{last_avail_idx} field is the free-running available ring
> +index where the device will read the next available head of a
> +descriptor chain.
> +
> +See also \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Available Ring}.
> +
> +\drivernormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Splited Virtqueues/ Virtqueue State}
> +
> +The driver SHOULD NOT access \field{Used State} of any splited virtqueues, it SHOULD use the
> +used index in the used ring.
> +
> +\devicenormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Splited Virtqueues/ Virtqueue State}
> +
> +The device SHOULD only accept setting Virtqueue State of any splited virtqueues
> +when DRIVER_OK is not set in \field{device status} or SUSPEND is set in \field{device status}.
> +Otherwise the device MUST ignore any writes to Virtqueue State of any splited virtqueues.

all these requests to ignore writes are to what end? just prohibit
driver from doing this.

> +
> +When SUSPEND is set, the device MUST record the Available State of every enabled splited virtqueue
> +in \field{Available State} field,
> +and correspondingly restore the Available State of every enabled splited virtqueue
> +from \field{Available State} field when DRIVER_OK is set.
> +
> +The device SHOULD reset \field{Available State} field upon a device reset.

At this point I have no idea
- how can a state of a virtqueue at a random time be represented
  by a 16 bit integer
- if it's not at a random time then why do you even need an integer -
  synchronize queue to memory and then all state is in memory



> -- 
> 2.35.3


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